The present invention is directed to the field of microelectronic packaging and, more particularly, to a gold plated solder material and a method of fluxless soldering attachment using the solder material.
Different types of components are commonly attached to substrates in microelectronic assemblies such as hybrid packages using eutectic attachment techniques. In a given hybrid package, different types of components are commonly attached to a substrate using different solder compositions. Typical metal alloy solder compositions that are used include gold-based alloys such as Au-Si, Au-Ge and Au-Sn. Such gold-based alloys do not oxidize in air and, consequently, do not require the use of fluxes to dissolve oxides and enable wetting of the joined surfaces. Fluxes are undesirable in soldering hybrid packages, however, due to the detrimental effects fluxes can have on certain components such as micro-optoelectronic devices.
Known gold-based solder compositions represent a range of eutectic temperatures. The different types of components in hybrid packages typically have upper temperature limits to which they can be heated during soldering without degrading the components. To avoid such degradation, the components are attached to the substrate using a soldering hierarchy approach, in which the components are soldered in the order of descending temperature resistance using different solder compositions in the order of descending processing temperature. For example, 96.4% Au-3.6% Si has a eutectic temperature of about 370.degree. C. and is typically heated to a processing temperature of about 390-420.degree. C. to form a eutectic bond between the component and the substrate. Components having an upper temperature limit above this processing temperature are attached first to the substrate using Au-Si solder. Au-Ge solder has a eutectic temperature below that of Au-Si solder and above that of Au-Sn solder. Accordingly, components having a temperature upper limit below the Au-Si solder process temperature range and above the Au-Sn process temperature are next attached to the substrate using the Au-Ge solder. This process is repeated until the soldering hierarchy is completed.
The present gold-based solders are less than completely satisfactory because they cannot be used in microelectronic assemblies such as hybrid packages to attach components having an upper temperature limit below the process temperature of Au-Sn solders. 80% Au-20% Sn solder has a eutectic temperature of about 280.degree. C. and typically is heated to about 310-330.degree. C. during soldering to ensure complete melting occurs. This processing temperature range can degrade some components.
Conventional solder compositions such as Sn-Pb solders have a sufficiently low process temperature for attaching such components, but these solders require fluxes to form a satisfactory joint. Accordingly, these solders are not suitable for use in hybrid packages that include micro-optoelectronic devices which can be degraded by fluxes.
Thus, there is a need for a solder composition and a method of using the solder composition to attach a component to a substrate in a microelectronic assembly such as a hybrid package that (i) does not require a flux; and (ii) melts at a relatively low temperature, making it suitable for attaching components at lower process temperatures than can be used for known gold-based solders.